Yeah Baumgarten, the K in a Rhombus or boxed K could be attributed to a Klett or possibly a Kelber. Axel E. gives that the jagged encircled K is that of a Klett but a workcard or similar will ferret out the demon. This example experienced proof between 1912 & early 1923 in Suhl. The script WK is that of master barrelsmith Wilhelm Kelber. But the boxed Ks denote a dual tube effort. With the high serial number I still wonder if it was sourced from Greifelt?

The last 2 images show the Imperial Eagle & Inspection stamp on the water-table.

Cheers,

Raimey
rse